Recently, there are increasing demands for further refinement of circuit patterns for increasing the degree of integration of a large scale integrated circuit device (hereinafter referred to as the LSI) realized by using semiconductor. As a result, it has become very significant to thin an interconnect pattern included in a circuit.
Now, the thinning of an interconnect pattern by a conventional optical exposure system will be described on the assumption that positive resist process is employed. In this case, a line pattern means a portion of a resist film not exposed to exposing light, namely, a resist portion (a resist pattern) remaining after development. Also, a space pattern means a portion of the resist film exposed to the exposing light, namely, an opening portion (a resist removal pattern) formed by removing the resist film through the development. In the case where negative resist process is employed instead of the positive resist process, the definitions of the line pattern and the space pattern are replaced with each other.
When a pattern is formed by using the optical exposure system, a photomask in which a light-shielding pattern of Cr (chromium) or the like is drawn in accordance with a desired pattern on a transparent substrate (a substrate having a transparent property) of quartz or the like is conventionally used. In such a photomask, a region where the Cr pattern exists is a light-shielding portion that does not transmit exposing light of a given wavelength at all (having transmittance of substantially 0%) and a region where no Cr pattern exists (an opening) is a transparent portion that has transmittance equivalent to that of the transparent substrate (having transmittance of substantially 100%) against the exposing light. At this point, all mask patterns are drawn on the transparent substrate, and in the pattern exposure, the transparent substrate is irradiated from a back side (i.e., a side where the mask patterns are not provided), and therefore, the mask patterns are irradiated with the exposing light having passed through the transparent substrate. Accordingly, when the transmittance of a mask pattern against exposing light is herein argued, the absolute transmittance of each portion of the mask pattern is not used but relative transmittance obtained on the basis of the transmittance of a transparent substrate against the exposing light (100%) is used.
In the case where the photomask as described above is used for the exposure of a wafer where a resist has been applied, an image of light having passed through the mask is projected onto the wafer. In this case, a light-shielding portion of the mask corresponds to an unexposed portion of the resist and an opening (transparent portion) of the mask corresponds to an exposed portion of the resist, so that a desired resist pattern can be formed on the wafer. Accordingly, such a photomask, namely, a photomask composed of a light-shielding portion and a transparent portion against exposing light of a given wavelength, is designated as a binary mask.
It is, however, difficult to accurately form a fine pattern smaller than the exposure wavelength (the wavelength of the exposing light) by using the binary mask because of the light diffraction phenomenon. This is for the following reason: Since the amplitude intensity of a diffraction image of light passing through the mask and projected onto a wafer is reduced, the proportion of zero-order light corresponding to non-diffracted light, namely, the proportion of a noise component in an optical image, is increased, and hence, a clear image cannot be obtained. As a result, a dimension error of a pattern provided on the mask is enhanced in the projected light image, which makes it difficult to form a pattern with desired dimensional accuracy. Such a phenomenon is designated as increase of MEF (mask error factor). In recent LSIs in which patterns are desired to be formed under highly accurate dimensional control, the reduction of the MEF is particularly a significant problem.
Therefore, a mask pattern having a function to shift a phase of light, namely, a photomask provided with a phase shifter (phase shifting mask), is recently used. In a phase shifting mask, a phase shifter for transmitting light in an opposite phase with respect to a transparent portion is used in a pattern region where a light-shielding portion is used in a conventional mask. Owing to this structure, zero-order light of light passing through a transparent portion can be cancelled through an interference effect by light in the opposite phase passing through the phase shifter, so that an optical image with high contrast can be formed. As a result, the increase of the MEF can be suppressed.
In the case where a phase shifter is used for forming a fine line pattern, the phase shifter preferably has transmittance as high as possible and ideally has transmittance equivalent to that of a transparent portion (100%). When a phase shifter with high transmittance is used for forming a thick line pattern, however, light in the opposite phase passing through the center of the phase shifter is unavoidably transferred onto a resist. Therefore, in order to simultaneously form a fine line pattern and a comparatively thick pattern, a halftone phase shifter (typically having transmittance of approximately 6%) for partially transmitting light in the opposite phase is used. Since the halftone phase shifter merely partially transmits light in formation of a comparatively thick pattern, the problem that the light in the opposite phase passing through the center of the phase shifter is transferred onto a resist can be avoided.
Furthermore, a photomask for simultaneously forming a fine line pattern and a comparatively thick pattern in which a phase shifter is used for forming a fine line pattern and a light-shielding portion is used for forming a comparatively thick pattern has been proposed (see, for example, Patent Document 1).
FIG. 36A is a diagram of a desired pattern to be formed by using a conventional photomask disclosed in Patent Document 1, and FIG. 36B is a plan view of the conventional photomask.
As shown in FIG. 36A, the desired pattern 20 to be formed on a wafer through exposure is composed of a fine line pattern and a thick pattern. Also, as shown in FIG. 36B, a transparent portion 14 is provided in a sufficiently large area on a transparent substrate 10, and a mask pattern composed of a light-shielding portion 11 and a phase shifter portion 13 is provided on the transparent portion 14 in a position corresponding to the desired pattern 20. It is noted that the transparent substrate 10 is perspectively shown in FIG. 36B. At this point, for forming a fine pattern with a width not larger than an exposing wavelength (a pattern A), the phase shifter portion 13 with high transmittance against exposing light is used, and for forming a pattern with a width sufficiently larger than the exposing wavelength (a pattern B), the transparent portion 11 or a phase shifter portion with low transmittance against the exposing light is used. In this manner, high contrast can be attained also in the formation of the fine pattern A with a width not larger than the exposing wavelength, and hence, the desired pattern 20 including the fine line pattern can be resolved. In other words, the desired pattern 20 can be formed with high resolution. Furthermore, in the formation of the pattern B with a width sufficiently larger than the exposing wavelength, the light-shielding portion 11 is used as a mask pattern, and therefore, the pattern formation can be satisfactorily performed while preventing degradation of contrast derived from a side effect of the light passing through the mask pattern.
Patent Document 1: Japanese Laid-Open Patent Publication No. 5-297565
Patent Document 2: Japanese Laid-Open Patent Publication No. 7-271013